chemistry form 6 sem 3 chapter 2

7/24/2019 Chemistry Form 6 Sem 3 Chapter 2

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CHEMISTRY FORM 6

ORGANIC CHEMISTRY

CHAPTER 2 : HYDROCARBON


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2.1 Nomenclature of ALKANE

Alkane is a saturated hydrocarbon as it contain only single bond in its

molecule

General formula for homologous series of alkane is CnH2n+2

Table below shows the naming of straight chain of alkane

Hydrocarbon which contain only

carbon-carbon single bond, CC

Hydrocarbon which contain at least one

carbon-carbon double bond, C=C or triple

bond, CC


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Name Molecular

formula

Molecular structure Name Molecular

formula

Molecular structure

Methane CH4 Ethane C2H6

Propane C3H8 Butane C4H10

Pentane C5H12

Hexane C6H14


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2.2 Naming alkane according IUPAC

Step 1 Step 2 Step 3 Step 4Find the longest

chain of carbon and

name accordingly. (it

does not has to be a

straight chain).

Identify the

branched carbon

(alkyl group) that

attached to the

main chain. Then,

name the alkyl

accordingly

CH3 methyl

CH3CH2 ethylCH3CH2CH2-propyl

Place a prefix upon

the similar alkyl

group (if any). If

there is 2 similar

alkyl, prefix diis

placed, if 3 similar

alkyl, prefix triis

placed.

State the position

where the branch is

located at which

carbon based on the

numbering gave

earlier.


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CH3(CH2)5CH3 CH(CH3)2C(CH3)3 CH3CH(CH2CH3)2 C(CH3)3CH2C(CH3)3

2,3-dimethylpentane 3-ethyl-3-

methylhexane

2,2,3-trimethylpentane 3,3-diethylhexane

3-methylpentane 3,5-dimethylheptane 3-ethyl-4-methylhexane3-ethyl-3,5-

dimethyloctane

n-heptane 2,2,3-trimethylbutane 3-methylpentane 2,2,4,4-tetramethylpentane


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Step 3 Complete the structure by placing one hydrogen (H)

atom at each of single bonds.


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Isomer of hexane, C6H14


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2.4 Physical properties of alkane

Alkane CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18

Boiling

point oC 162 8.6 42.2 0.5 36.3 68.7 98.4 126

Boiling pointtrend

Density

(g/cm3)-- -- 0.50 0.58 0.63 0.66 0.68 0.70

Density trend

SolubilityNot soluble in ..

Soluble in

BOILING POINT INCREASE DOWN HOMOLOGOUS SERIES

DENSITY INCREASE DOWN HOMOLOGOUS SERIES

water

organic solvent


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A) Boiling point of alkenes

The boiling point when going down to homologous series of

alkane.

All alkane possessed the same intermolecular forces : weak

forces

Greater the , stronger the forces, the boiling point

Boiling point of isomers of the same molecular formula varies with the

branched molecules

Straight chain has .... boiling point compared to branched chain asstraight chain molecule has higher compared to a

branched chain. The positioning of alkyl and number of alkyl also effect

the boiling point of alkane. 2-methylpentane as a higher boiling point than

3-methylpentane as it has a greater exposure of intermolecular forced

increase

Van Der Waals

molecular massweak Van Der Waalsincrease

higher

total surface area


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B) Solubility of alkane

All alkanes are often consider as

molecule as the dipole of moment created in molecule is verysmall.

Since alkane is . Molecule, it dissolve easily in

non-polar solvent such as benzene, and ether.

Alkane does not form bond in water, so it is

in water. Thus, alkane is also described as

. (waterhating).

The longer the alkane chain, the more insoluble it is in water.

non-polar

non-polar

hydrogen

insolublehydrophobic


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2.5 Chemical Properties of Alkane

2.5.1 Preparation of Alkane

Alkane can be prepared using the following methods :

Decarboxylation of sodium salt of a carboxylic acid

RCOOH + NaOH RH + Na2CO3

Example :

Kolbes method : electrolysing concentrated sodium ethanoate

Cathode : 2 H2O + 2 e- H2 + 2 OH

-

Anode : 2 CH3COO-

C2H6 + 2 CO2 + 2 e-

Wurtz reaction : reaction of sodium on alkyl halide in ether.

2 RX + 2 Na R R + 2 NaX

Example

CH3COOH + 2 NaOH CH4 + H2O + Na2CO3

2 CH3CH2Cl + 2 Na CH3CH2CH2CH3 + 2 NaCl


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2.5.2 Reaction of Alkane

Since alkane is a . hydrocarbon, so alkane is inert to most of

the chemical reaction

Table below shows the description of reaction of ethane with other

substances.

From the series of reaction above it can be conclude that

Ethane does no react with polar or ionic substances

Ethane react with non-polar substances such as Cl2 , Br2 and O2 andenergies are required for reaction to occur.

Reagents Effect on ethane

Sodium hydroxide aqueous No effect on hot or cold condition

Concentrated hydrochloric acid No effect on hot or cold condition

Acidified potassium manganate (VII) No effect on hot or cold condition

Air (oxygen) No effect under room condition. Burns when heated

Bromine water No effect on dark. Decolourised slowly under sunlight

Chlorine gas No effect on dark. Reaction occur under sunlight

saturated


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1. Combustion of alkanes

All hydrocarbon react with oxygen to form carbon dioxide and water.

The equation for a complete combustion for all hydrocarbons can be

represented by the equation

C2H6

C5H12

C8H18

Note that the reaction is exothermic for all hydrocarbons. Equation above

is also known for Hc. Higher the number of carbon, the more exothermic

the reaction.

Under limited supply of air (oxygen), sometimes, carbon monoxide (CO) is

produced instead of CO2.

mol/kJmHOH

2

yCOxO

4

yxHC 222yX =+

++

C2H6 + 7/2 O2 2 CO2 + 3 H2O

C5H12 + 8 O2 5 CO2 + 6 H2O

C8H18 + 25/2 O2 8 CO2 + 9 H2O


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2.Halogenation of alkanes

When alkane is run together with chlorine gas under the presence of

ultraviolet ray (which comes naturally from sunlight)

Example : CH4 (g) + Cl2 (g) CH3Cl (g) + HCl (g)

C2H6 (g) + Cl2 (g)

The mechanism for the reaction of chlorination of alkane can be explainedusing the following steps

Step 1 : Initiation Step 2 : Propagation Step 3 : Termination

Cl Cl 2 Cl H = +242 kJ/mol

H3CH CH3 + H

H = + 433 kJ/mol

Since ...................... required lower

energy to form radical, so the initiation

will start off with .. Gas

Since chlorine radical are highlyreactive, when it collide with methane

molecule forming HCl

and methyl radical

H3CH + Cl H3C + HCl

Methyl radical will propagate with other

chlorine molecule and

forming back chlorine radical

H3C + ClCl H3CCl + Cl

Under such propagation reaction

thousands of methane and chlorine

molecules will react continuously

When 2 free radicals collide with eachother and combined, the reaction stops.

This reaction is highly exothermic,

where

H3C + Cl H3CCl

H = -349 kJ/mol

H3C + CH3 H3CCH3(H = -368 kJ/mol

Usually, termination will occur when

[radical] > [molecule], which is after

thousands of propagation.

The presence of small amount of

ethane may also present due to the

collision between 2 methyl radicals

chlorine

chlorine


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2.1.1 Sources of hydrocarbon

The main sources of hydrocarbons are :a) crude oil b) coal

c) natural gas

Since all these main sources are made up from dead animals and plants,

so they are also known as

Coal is complex mixture consisting mainly hydrocarbons, which is mainly

made up from dead plaints in swamp.

Petroleum is a mixture of hydrocarbons (alkanes, alkenes, alkyne), whilenatural gas contain mainly . and some

The mixture in petroleum can be separated by using

. in oil refinery. Diagram below shows the chamber

and oil refinery used to separate the mixture of petroleum.

fossil fuel

methane ethane

fractional distillation


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Fractional

distillation Products Uses

Petrol gas Use for house cooking gas

Gasoline Use as fuel for automobile vehicle

Naphtha Use to synthesis different petrochemical

Diesel oil Use as fuel of heavy vehicle such as busor lorry

Kerosene Use as fuel for jet engine and oil stove

Lubricant Oil

Use for lubrication, making wax and polish

Fuel Oil Fuel for ship and power station

Bitumen (asphalt)Use as tar for paving road surface and

coating underground water pipe


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The separation does not end with fractional distillation. They are then

treated with various ways to improve the quality and quantity of useful

hydrocarbon. One of the major treatments gives after fractional distillationis cracking process.

Cracking of hydrocarbon

Thermal cracking (Pyrolysis) Catalytic cracking

Using high temperature, bond breaking

(homolytic fission) take place and form

various products of unbranched alkane

and alkene Example, when breaking decane, C10H22

C10H22 C3H6 + C7H16C10H22 C4H8 + C6H14

With the aid of zeolite as catalyst, carbon

cracking can occur at lower temperature

compare to thermal cracking.

Products using catalytic cracking usuallycontain branched alkane and alkene.

C10H22


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2.7 Cycloalkane (alicyclic compound)

Cycloalkane has a general formula of CnH2n

Some examples of cycloalkane

Cycloalkane Molecular formula Displayed formula Skeletal formula

Cyclopropane

Cyclobutane

Cyclopentane

Cyclohexane

C3H6

C4H8

C5H10

C6H12


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2.7.1 Naming cycloalkane

The way of naming cyclolalkane is more or less the same with naming

alkane. If theirs is one alkyl attached to the cycle, it will be automaticallybecome 1 by itself. E.g. methylcyclobutane

(not 1-methylcyclobutane)

If theres more than one group attaching the cycle, only then numberingwill be given to the particular number of C that it is attached.

methylcyclopropane

3-ethyl-1-methylcyclopentane

1,2,4-trimethylcyclohexane

1,2,3-trimethylcyclooctane

3-ethyl-2-methyl-1-

propylcyclobutane


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2.7.2 Preparation and Reaction of Cycloalkane

Cycloalkane can be prepared by catalytic hydrogenation of benzene at

200oC

Reaction of cycloalkane is similar to alkane. When react with chlorine /bromine gas under sunlight, substitution reaction take place

Mechanism :

Initiation

Propagation

Termination


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2.8 Alkene Nomenclature of alkenes and cycloalkenes

The homologous series of alkenes has general formula of CnH2n.

The significance of alkene is all of them have C=C in their molecules with

its name end with ene

NameMolecular

formulaMolecular structure Name

Molecular

formulaMolecular structure

Ethene C2H4 Propene C3H6

Butene C4H8

Pentene C5H10

Hexene C6

H12

But-2-ene But-1-ene

pent-2-ene pent-1-ene

Hex-1-ene Hex-2-ene Hex-3-ene


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In naming alkene, the following steps are given

Step 1 : Find the longest C C chain which contain double bond in it

(parent chain) and name them

Step 2 : Find and name the alkyls attached to the parent chain.

Step 3 : If there are more than 2 of the same type alkyls, prefix are put

accordingly.Step 4 : Put the number of the alkyl that attached to the particular

carbon atom.

Example : Name the following alkenes accordingly


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2-methylbut-2-ene 2-ethyl-3-methylpent-1-ene 3,4-dimethylhex-3-ene

2-methylpropene 2,3-dimethylpent-2-ene 3,5-dimethylhept-3-ene


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2.8.1 Naming alkene with more than one single bond & cycloalkene

A diene (alkene with 2 C=C bond) and cycloalkene has general formula

of CnH2n2. In diene, the position of both C=C in parent chain has to be stated in alkan-

x,y-diene, whereas in cycloalkene, C=C is always place as C1=C2. So the

numbering is fixed for naming.

Example, name the following diene / cycloalkene below

2-methylbut-1,3-diene2,5-dimethylhex-1,3-diene oct-2,5-diene

3-methylcyclopropene3-ethyl-2-methylcyclohexene

3,4,5-trimethylcyclopentene


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2.9 Isomerism in alkene.

Alkenes which contain at least 4 Carbon atoms may exhibit 2 isomerism,

structural and stereoisomerism.

For example, butane (C4H8) contain 5 isomers.


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Isomers of pentene


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2.10 Physical Properties of Alkene

A) Boiling Point of Alkene

The boiling point when going down to homologous series ofalkane.

All alkane possessed the same intermolecular forces : weak

forces

Greater the .., stronger the forces,

the boiling point

Alkene C2H4 C3H6 C4H8 C5H10 C6H12 C7H14 C8H16 C9H18

Boiling

point oC 164 12.0 5.8 0.5 38.0 72.07 96.5 117

Boiling

point trendSolubility in

water

Boiling point increase

Insoluble in water (solubulity decrease)

increase

Van Der Waals

molecular mass weak Van Der Waals

higher


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2.11 Preparation of Alkene

Alkene can be prepared in a few ways

Name of

reaction

Reagent used

and conditionEquation

Dehydro-

halogenationfrom

haloalkane

Ethanolic

sodiumhydroxide (heat

& reflux)

Dehydration

(removal of

water)from

alcohol

Excess conc.

H2SO4at 1800C

orAlumina (Al2O3)

at 350oC


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2.12 Chemical reaction of alkene

Name of

reaction

Reagent used and

condition

Equation

Hydrogenation

Hydrogen gas under

--------------

Nickel (Ni) at 180oC

@Platinum (Pt) at

room temperature

CH3CH=CH2 + H2 (g) CH3CH2CH3 (g)

propene propane

cyclohexene cyclohexane

Halogenation

Halogen gas, X2

(X2 = Cl2 ; Br2 ; I2)

Addition of

Hydrogen

halide

Hydrogen halide

( H X )

(X = Cl ; Br ; I)

Ni

Name of Reagent used and


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Name of

reaction

Reagent used and

conditionEquation

Hydration

Steam (H2O)

---------

Phosphoric acid,

(H3PO4 )

At 300oC ; 60 atm

Hydroxylation

(cold, diluted

acidified

KMnO4)

KMnO4 (aq) / H+

(cold and diluted)

Oxidation

(under hot,

concentrated

acidified

potassium

manganate

(VII)

KMnO4 (aq) / H+

(hot &

concentrated)


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2.12 Chemical reaction

(1) Hydrogenation of alkene

Carry out under mixture of alkene and hydrogen over a finely dividedtransition metal as a catalyst.

2 catalysts can be used in hydrogenation

i) Platinum : ~ can react even under room condition. Longer alkene required

some heat

ii) Nickel : ~ required high temperature to allow hydrogenation to occur

(180oC)

Hydrogenation is an exothermic reaction and its H is about 120 kJ / mol CH3CH=CH2 (g) + H2 (g) CH3CH2CH3 H = 124 kJ / mol

Catalytic hydrogenation is important in food industries especially in

hardening unsaturated fats and oil to make margarine. Unsaturated

hydrocarbon makes them too soft for commercial use.

CH3(CH2)7CH=CH(CH2)7COOH + H2 (g) CH3(CH2)16COOH

In industries, a special Raney Catalyst is used to replace platinum as it is

EXPENSIVE!!!

(2) H l ti f lk


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(2) Halogenation of alkene

Chlorine and bromine react readily with alkene and form dichloroalkane

and dibromoalkane respectively. Cl2 and Br2 gas are add across doublebond.

CH3CH=CH2 (g) + Cl2 (g) CH3CH(Cl)CH2Cl

The mechanism of halogenation can be explained by a few steps describe

below :

Step 1 : Formation of carbocation propene has region of high electron

density because of the electron. When Cl2 approaches, molecule is

strongly polarised by region and consequently formed an induce dipole.

The positive charge end of Cl2 molecule act as electrophile and bond to

C=C via electroplilic addition and caused Cl+Cl repelled. As a result,

carbocation & chloride ion are formed.


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Step 2 : Nucleophilic attack to form addition product carbocation

formed is very unstable. It quickly combines with Cl ion to produce by

heterolytic fission of Cl2 molecule to give 1,2-dichloropropane.

However, if bromine wateris used instead of bromine gas, the results of

products are not as same as in bromine gas. When bromine water is

reacted with propene

( ) f


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(3) Addition of hydrogen halide

Unlike addition of halogen, addition of hydrogen halide produced 2

products. For example, when propene react with hydrogen bromide (HBr)CH3CH=CH2 + HBr CH3CH2CH2Br + CH3CH(Br)CH3

Propene 1-bromopropane 2-bromopropane

(minor) (major) The major / minor product of the reaction can be predicted using

Markovnikoffs Rule where it stated when an unsymmetrically substituted

alkene reacts with a hydrogen halide, the hydrogen adds to the carbon

that has the greater number of hydrogen substituents, and the halogenadds to the carbon having fewer hydrogen substituents.

S 1 El hili k h h l h d b id


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Step 1 : Electrophilic attack when the polar hydrogen bromide

approaches propene, the positively charged hydrogen ion is polarising

C=C, and caused Br

to form

Step 2 : Nucleophilic attack the negative bromide ion react fast with the

unstable carbocation.

+

R l ti t bilit f b ti b l i d i M k ik ff


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Relative stability of carbocation can be explained using Markovnikoffs

Rule. According to the rule, a tertiary (30) carbocation is more stable than

a secondary (20

) carbocation than a primary (10

) carbocation. this is due tothe inductive effect of the electron-donating alkyl group.

In the example above, there are 2 methyl group donating electron to

positive charged carbon electron at 20 carbocation whereas there are 1

ethyl group in 10 carbocation donating electron to the positively chargedelectron.

As a result, 20 carbocation are more stable as the 2 alkyl group tend to

decrease the charge density of C, making the cation more stable.

stability of carbocation increase.

(4) H d ti ( dditi f t ) i lk


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(4) Hydration (addition of water) in alkene

Using phosphoric acid as acidic medium, hydration of alkene can be

represent by equation :CH3C(CH3)=CH2 + HOH CH3CH(CH3)CH2OH + CH3C(CH3)(OH)CH3

(minor) (major)

2-methylpropene 2-methylpropan-1-ol 2-methylpropan-2-ol

Similar to hydrogen halide, hydration of alkene follows Markovnikoffs Rule.

The mechanism of hydration of alkene is slightly different from addition of

hydrogen halide

Step 1 : Protonation of the carboncarbon double bond in thedirection that leads to the more stable carbocation

Step 2 : Water acts as a nucleophile to capture carbocation


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Step 2 : Water acts as a nucleophile to capture carbocation

Step 3 : Deprotonation of tert-butyloxonium ion. Water acts as a Brnsted Lowry base:

Other than using diluted acid medium, sometimes, hydration of alcohol isprepared by adding concentrated sulphuric acid to alkene.

When H2SO4 (conc) is added to alkene under room condition, it give analkyl hydrogensulphate


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Hydrolysis of alkyl hydrogensulphate will convert into alcohol

(5) Oxidation of alkene using acidified potassium manganate (VII)


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(5) Oxidation of alkene using acidified potassium manganate (VII)

Alkene are readily oxidised by acidified KMnO4 (decolourised the purple

colour of KMnO4) and give different products under different condition If cold diluted acidified KMnO4 is used, a diol is given as a product.

If hot concentrated acidified KMnO4 is used, a ketone or an aldehyde

is formed which will further oxidised to become a carboxylic acid or into

carbon dioxide and water depend on alkene.

a) Hydroxylation of alkene (react under cold dilute acidified KMnO4)

The product of this reaction is a diol (di-alcohol) which contain 2

hydroxyl group.

This reaction is often used to distinguish between saturated hydrocarbon

and unsaturated hydrocarbon (alkane and alkene)

b) Oxidation of alkene using hot concentrated acidified potassium


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b) Oxidation of alkene using hot, concentrated acidified potassium

manganate (VII)

When alkene react with hot concentrated acidified potassium manganate(VII), it will oxidise immediately to form aldehyde or ketone, depend on the

type of alkene

Using this method, the position of C=C in alkene can be deduced. If the

alkene is a 10 alkene, it will turn lime water chalky when the particularalkene is reacted with hot concentrated acidified potassium manganate

(VII)


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Alkene Products

methanal

Methanoic acid

a CH3CH2CH=CHCH3 + H2 (g) CH3CH2CH2CH2CH3


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a.CH3CH2CH CHCH3 + H2 (g)

b.CH3CH2CH=CH2 + Cl2 (g)

c. CH3CH=C(CH3)CH3 + Br2 (l)

d.CH3CH(CH3)CH=CH2 + HCl (g)

CH3CH2CH2CH2CH3

CH3CH2CHClCH2Cl

CH3CHBrC(CH3)(OH)CH3 major

CH3CHBrCBr(CH3)CH3 minor

CH3CH(CH3)CHClCH3 major

CH3CH(CH3)CH2CH2Cl minor


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4-ethyl-2,2,4-trimethylhexane

2,2,4,5-tetramethylhexane

5-ethyl-3,4-dimethyloctane


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2,3,4,6,6-pentamethyl-3-heptene

7-ethyl-1,3-dimethylcyloheptene

C(CH3)2=C(CH2CH3)CH(CH3)CH(CH3)2

CH2=CHC(CH3)(CH2CH3)C(CH3)=CH2

Isomers of pentene


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Isomers of pentene

Practice : Write the chemical equation for the following reaction


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Practice : Write the chemical equation for the following reaction

1. Butane react with chlorine under the presence of sunlight

CH3CH2CH2CH3 + Cl2 CH3CH2CH2CH2Cl + HCl

2. Pentane burned with excess air

C5H12 + 8 O2 5 CO2 + 6 H2O

3.Octane burned with excess airC8H18 + 25 / 2 O2 8 CO2 + 9 H2O

4.Propene reacts with hydrogen gas using platinum as catalyst

CH3CH=CH2 + H2 CH3CH2CH35. 1-hexene burned with excess air

C6H12 + 9 O2 6 CO2 + 6 H2O

6. 2-heptene reacts with bromine water

CH3CH2CH2CH2CH2CH=CH2 + Br2 + H2O

CH3CH2CH2CH2CH2CH(OH)CH2Br + CH3CH2CH2CH2CH2CHBrCH2Br

7.Propene reacts with hydrogen chloride

CH3CH=CH2 + HCl CH3CH2CH2Cl (min) + CH3CHClCH3 (maj)

8. 1-Butene react with excess oxygen


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C4H8 + 6 O2 4 CO2 + 4 H2O

9.2-Pentene reacts with steam catalysed by sulphuric acidCH3CH=CHCH2CH3 + H2O CH3CH(OH)CH2CH2CH3 CH3CH2CH(OH)CH2CH3

10. 3-Hexene reacts with cold dilute acidified KMnO4

CH3CH2CH=CHCH2CH3 + KMnO4/H+ CH3CH2CH(OH)CH(OH)CH2CH3

11. 2-methylhex-2-ene reacts with cold dilute acidified KMnO4

CH3C(CH3)=CHCH2CH2CH3 + KMnO4/H+

CH3C(CH3)(OH)CH(OH)CH2CH2CH3

12. Propane react with fluorine under the presence of sunlight

CH3CH2CH3 + F2 CH3CH2CH2F + HF

13. Propene is polymerized at 2000C and 1200 atm

14. 2-methylbut-2-ene react with bromine water under the presence of

sunlight.

4. Proposed the mechanism for the following reaction below


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chemistry form 6 sem 3 chapter 2

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